In recent years, extensive studies have been made on DDS carriers (carriers) for delivering a physiologically active substance to an intended site in a required amount. Attention has now been paid to a stimuli responsive carrier from the standpoint of an improvement in accumulation and the selection of the delivery site, and many reports have been made on studies concerned with external stimulation with heat, a magnetic field or the like, and in vivo stimulation such as of molecular recognition, pH change, enzymatic reaction or the like. Among them, a carrier which is sensitive to a weakly acidic pH has been investigated for a long time.
For instance, as a pH sensitive carrier responsive to a weakly acidic environment, there are known pH sensitive liposomes wherein various types of pH sensitive elements such as PHC (Palmitoyl Homocysteine), oleic acid, CHEMS (Cholesteryl Hemisuccinate) and the like are added to liposomes containing PE (Phosphatidylethanolamine) (S. Simoes et al., Adv. Drug Deli. Rev. 2004 56 947-965 and the like). Recently, there have been reported studies, for the purpose of enhancing function, on novel synthetic materials such as PEAA (Poly(2-ethylacrylic Acid)), SucPG (Succinylated Poly(glycidol)) and the like, synthetic peptides such as GALA, pHLIP (pH Low Insertion Peptide) and the like, biodegradable materials such as PLGA (Poly(Lactic-co-glycolic Acid)) and the like, and VLP (Virus Like Particle) and Virosome using a pH sensitive viral component.
The pH sensitive carrier is expected to efficiently deliver a physiologically active substance to a site such as of a tumor or inflammation where a pH lowers in a living body (Reshetnyak et al., PNAC 2007 vol. 104, 19, 7893-7898) or to deliver to the cytosol by utilizing the acidification of vesicles after being taken up by cells.
With respect to the delivery to the cytosol by using the acidification of vesicles, it has been found that migration of a drug to the cytosol is facilitated by promoting membrane fusion of a carrier upon delivery to endosomes, and a DDS carrier of a peptide derivative having a pH sensitive site whose structure is changed in response to the pH, a membrane fusion site and a transmembrane site has been reported in Japanese Patent Laid-open No. 2006-34211.
Such a delivery of a physiologically active substance to vesicles is utilizable in various fields and is thus a technique that is in demand. For instance, delivery of RNA or DNA to the cytosol enables gene therapy, and it has been expected to induce CTL (Cytotoxic T Lymphocyte) by the delivery of an antigen to the cytosol. The cytosolic delivery of a low molecular weight anticancer agent has been reported as having an improved effect on activity, and its application to a novel type of drug for use as an intracellularly targeted drug has been considered. The cytosolic delivery of a physiologically active substance is a major problem to solve in these fields and is thus desirable.
In view of the fact that there is a successful case of delivering pHLIP having sensitivity to a pH of not higher than 6 to an acidosis site as described in Reshetnyak et al., PNAS 2007 vol. 104, 19, 7893-7898 or a pH arriving within endosomes in the vicinity of 4 as described in S. Simoes et al., Adv. Drug Deli. Rev. 2004 56 947-965, it is required that a pH-sensitive carrier have high sensitivity between neutral pH and a pH of 4. This is described in many documents. It is further important from the standpoint of practical use that the carrier is made of a safe material.